GPT5 on Everett's Many Worlds

[John Clark]

I asked GPT5, which just came out yesterday, the following question:

"How would Everett's Many World's idea explain the way the two slit experiment works and the key role that "which way" information plays in it?"

And this is what he she or it said: 

==============

Okay- let's put on our Everett hat and walk through the two-slit experiment step by step, with the “many worlds” interpretation (MWI) in mind.

---

1. What Everett says about quantum stuff

In Everett’s picture, the wavefunction is real and never collapses.
Instead of “the particle chooses a path when you measure,” all possible outcomes actually happen — but in different, non-communicating branches of reality.

The key: the Schrödinger equation rules everything, all the time. No special “measurement” magic, just physics.

2. Two-slit without which-way info

Imagine you have a photon heading toward two slits.

• Before the slits:
  The photon’s wavefunction spreads so it has an amplitude to go through slit A and an amplitude to go through slit B.
  In MWI terms, there’s still just one “world,” but in that world the photon is in a superposition of both paths.

• After the slits:
  These two path components overlap and interfere, producing the interference pattern on the detection screen.
  Since there’s no device or environmental disturbance storing “which-way” info, the two parts of the wavefunction can still affect each other.

In MWI language: there is no branching here — both possibilities are still part of one combined world and can interfere.

3. Two-slit with which-way info

Now add a detector at the slits that becomes correlated with the photon’s path.

• If the photon goes through slit A → detector state changes to “A detected.”

• If through slit B → detector state changes to “B detected.”

Everett’s take:
This doesn’t collapse the photon’s wavefunction — instead, the photon + detector become entangled.
The total wavefunction now has two components:

Branch 1: Photon through A  + Detector says “A”
Branch 2: Photon through B  + Detector says “B”

These are different worlds: in one, the detector reads “A”; in the other, “B.”
Because the detector states are physically different and don’t overlap, the photon’s A-path wave and B-path wave can no longer interfere — they “live” in separate worlds.

---

4. Why which-way info kills the interference

From the MWI perspective:

• No which-way info: The two paths are in the same world and can interfere.

• Which-way info exists (even if nobody looks at it): The environment has branched into distinct versions, each containing only one path. Interference requires the waves to be in the same branch, so once branching occurs, interference is gone.

This matches decoherence: the environment records path info, which makes branches orthogonal (no overlap → no interference).

5. The “Everett moral”

In Many Worlds, nothing ever really collapses. What we call “measurement” is just branching into non-communicating worlds.
The two-slit experiment is then:

• No measurement → no branching → interference visible.

• Measurement → branching → no interference between branches.

============

Me: "Thanks GPT, you are the man! Or rather you are the AI.

GPT5: "Haha — I’ll take “the AI” as a compliment! Glad I could help make Everett’s worlds a bit clearer."

John K Clark

[Brent Meeker]

So did you ask it how MWI deals with continuous splitting, as in detecting the decay of radioactive atoms?

Brent

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[John Clark]

On Fri, Aug 8, 2025 at 4:57 PM Brent Meeker <meeke...@gmail.com> wrote:

> So did you ask it how MWI deals with continuous splitting, as in detecting the decay of radioactive atoms?

No I did not. However on November 17, 2024 I sent the following to this list that was about Many Worlds and radioactive decay: 

====================== 

"Nobody needed an interpretation of classical physics, nobody needed a definition of "measurement" or "observer" because regardless of the old cliché about a watched pot never boiling, the time it takes to boil a pot of water really doesn't change depending on if you are watching it or not, but in the weird quantum world you really CAN delay the decay of a radioactive atom if you watch it closely enough, and Many Worlds has no problem explaining how this "Quantum Zeno Effect" works.   

Suppose an atom has a half-life of one second and I'm watching it, the universe splits and so do I after one second. In one universe the atom decays and I observed that the atom has decayed, in the other universe the atom has not decayed and I observed that it has not decayed. 

In the universe where the atom didn't decay after another second the universe splits again, and again in one universe it decays but in the other it has not, it survived for 2 full seconds. So there will be a version of me that observes this atom, which has a one second half-life, surviving for 3 seconds, and 4 seconds, and 5 years, and 6 centuries, and you name it. By utilizing a series of increasingly complex and difficult procedures it is possible for the lab (and you) to be in the universe that contains labs and versions of you that see the atom surviving for an arbitrarily long length of time. But the longer the time past its half-life the more splits are involved, and the more difficult the experiment becomes.  Soon it becomes ridiculously impractical to go further, but it's not impossible. 

Quantum Zeno effect 

The useful role that Many Worlds provides is that it doesn't need to explain what a "measurement" or an "observer" is, nor does it need to explain exactly, or even approximately, where the Heisenberg cut is. And it doesn't need to explain what consciousness is because it has nothing to do with it." 

===============

John K Clark    See what's on my new list at  Extropolis

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[Brent Meeker]

On 8/8/2025 2:50 PM, John Clark wrote:

On Fri, Aug 8, 2025 at 4:57 PM Brent Meeker <meeke...@gmail.com> wrote:

> So did you ask it how MWI deals with continuous splitting, as in detecting the decay of radioactive atoms?

No I did not. However on November 17, 2024 I sent the following to this list that was about Many Worlds and radioactive decay: 

====================== 

Suppose an atom has a half-life of one second and I'm watching it, the universe splits and so do I after one second. In one universe the atom decays and I observed that the atom has decayed, in the other universe the atom has not decayed and I observed that it has not decayed. 

In the universe where the atom didn't decay after another second the universe splits again, and again in one universe it decays but in the other it has not, it survived for 2 full seconds. So there will be a version of me that observes this atom, which has a one second half-life, surviving for 3 seconds, and 4 seconds, and 5 years, and 6 centuries, and you name it. 

And in fact for every value of t>0 in R.  Another fanciful result of MWI.  And all just to avoid dealing with a definite result.

Brent

[John Clark]

On Fri, Aug 8, 2025 at 10:23 PM Brent Meeker <meeke...@gmail.com> wrote:

>> Suppose an atom has a half-life of one second and I'm watching it, the universe splits and so do I after one second. In one universe the atom decays and I observed that the atom has decayed, in the other universe the atom has not decayed and I observed that it has not decayed. 
In the universe where the atom didn't decay after another second the universe splits again, and again in one universe it decays but in the other it has not, it survived for 2 full seconds. So there will be a version of me that observes this atom, which has a one second half-life, surviving for 3 seconds, and 4 seconds, and 5 years, and 6 centuries, and you name it. 

> And in fact for every value of t>0 in R.

Yep.

  >Another fanciful result of MWI. 

What I have described in the above that was, as you say, obtained as a result of MWI, is called the "Quantum Zeno Effect" and it is NOT fanciful, in 1990 it was CONFIRMED EXPERIMENTALLY to exist. 

Quantum Zeno effect

Some people claim that Many Worlds is not scientific because it is not falsifiable, but if the above lab experiment had turned out differently and it had found no evidence of the Quantum Zeno Effect then that would have proven that Many Worlds was wrong. But when I say it's not fanciful that it's possible to delay the decay of a radioactive atom arbitrarily longer than its half-life, that doesn't mean it's practical as I made clear in my original post: 

"By utilizing a series of increasingly complex and difficult procedures it is possible for the lab (and you) to be in the universe that contains labs and versions of you that see the atom surviving for an arbitrarily long length of time. But the longer the time past its half-life the more splits are involved, and the more difficult the experiment becomes.  Soon it becomes ridiculously impractical to go further, but it's not impossible." 

> And all just to avoid dealing with a definite result.

Definite result? All radioactive atoms seem to have a definite half-life that you can look up in any physics textbook, however that textbook is making the implicit assumption that you are not observing those atoms very very carefully; and normally that would be a reasonable assumption to make, but not if you are a very skilled experimenter trying to find out more about the nature of quantum reality.  

 John K Clark    See what's on my new list at  Extropolis

5d5

[Brent Meeker]

On 8/9/2025 4:05 AM, John Clark wrote:

On Fri, Aug 8, 2025 at 10:23 PM Brent Meeker <meeke...@gmail.com> wrote:

>> Suppose an atom has a half-life of one second and I'm watching it, the universe splits and so do I after one second. In one universe the atom decays and I observed that the atom has decayed, in the other universe the atom has not decayed and I observed that it has not decayed. 
In the universe where the atom didn't decay after another second the universe splits again, and again in one universe it decays but in the other it has not, it survived for 2 full seconds. So there will be a version of me that observes this atom, which has a one second half-life, surviving for 3 seconds, and 4 seconds, and 5 years, and 6 centuries, and you name it. 

> And in fact for every value of t>0 in R.

Yep.

  >Another fanciful result of MWI. 

What I have described in the above that was, as you say, obtained as a result of MWI, is called the "Quantum Zeno Effect" and it is NOT fanciful, in 1990 it was CONFIRMED EXPERIMENTALLY to exist. 

And the quantum Zeno effect is about atoms NOT decaying.  There is no experimental support for a continuum branching of worlds, that's what is fanciful.  I don't know why you tried to change the question to on of NOT decaying.

Brent

"By utilizing a series of increasingly complex and difficult procedures it is possible for the lab (and you) to be in the universe that contains labs and versions of you that see the atom surviving for an arbitrarily long length of time. But the longer the time past its half-life the more splits are involved, 

Independent of the time past or the value of the half-life the number of splits, according to MWI, is the same as the number of points on a line, i.e. infinite.

and the more difficult the experiment becomes.  Soon it becomes ridiculously impractical to go further, but it's not impossible." 

> And all just to avoid dealing with a definite result.

Definite result? All radioactive atoms seem to have a definite half-life 

The "definite half-life" is a parameter in the probability distribution.

that you can look up in any physics textbook, 

Yes, that's something you should try.

Brent

[John Clark]

On Sat, Aug 9, 2025 at 3:51 PM Brent Meeker <meeke...@gmail.com> wrote:

>> Me: Suppose an atom has a half-life of one second and I'm watching it, the universe splits and so do I after one second. In one universe the atom decays and I observed that the atom has decayed, in the other universe the atom has not decayed and I observed that it has not decayed. 

In the universe where the atom didn't decay after another second the universe splits again, and again in one universe it decays but in the other it has not, it survived for 2 full seconds. So there will be a version of me that observes this atom, which has a one second half-life, surviving for 3 seconds, and 4 seconds, and 5 years, and 6 centuries, and you name it. 

         

>Another fanciful result of MWI.

 

> Me: What I have described in the above that was, as you say, obtained as a result of MWI, is called the "Quantum Zeno Effect" and it is NOT fanciful, in 1990 it was CONFIRMED EXPERIMENTALLY to exist. 

 

> And the quantum Zeno effect is about atoms NOT decaying. 

Obviously I already knew that because I had just explained it! Is my explanation the first time you've heard of the Quantum Zeno Effect? 

 

> There is no experimental support for a continuum branching of worlds, that's what is fanciful. 

Immediately after I went into some detail explaining what the Quantum Zeno Effect was, you commented that what I was saying was a "another fanciful result of MWI". But it's not fanciful that the Quantum Zeno Effect exists because it's been experimentally confirmed to exist, and it's not fanciful that it can be derived from Many Worlds, in fact if experimenters had found that the Quantum Zeno Effect did not exist then Many Worlds would've been proven to be wrong.  

> I don't know why you tried to change the question to on of NOT decaying.

You really don't know? I didn't change the subject because the relationship between observation and a quantum state decaying or not decaying is what the Quantum Zeno Effect is all about. And I'm not the one who brought up the subject in the first place, you are. You wanted to know if I had asked GPT5 about "how MWI deals with continuous splitting, as in detecting the decay of radioactive atoms" and I responded that I didn't need to ask because I already knew the answer and had written a post about that very subject about a year ago.

>> "By utilizing a series of increasingly complex and difficult procedures it is possible for the lab (and you) to be in the universe that contains labs and versions of you that see the atom surviving for an arbitrarily long length of time. But the longer the time past its half-life the more splits are involved, 

> Independent of the time past or the value of the half-life the number of splits, according to MWI, is the same as the number of points on a line, i.e. infinite.

We've been over this many many times before as I'm sure you remember, and yet you STILL bring out that same silly argument even though you must know by now it's invalid. 

1) Many Worlds does NOT insist that the number of worlds is equal to the number of points in a line. 

2) Many Worlds does NOT insist that the number of worlds is equal to the number of countable integers.

3) Many Worlds does NOT insist that the number of worlds is an infinite number at all.

4)  Many Worlds does NOT say that probabilities can be obtained by counting branches. 

 John K Clark    See what's on my new list at  Extropolis

eaz

 

[Brent Meeker]

On 8/9/2025 2:53 PM, John Clark wrote:

On Sat, Aug 9, 2025 at 3:51 PM Brent Meeker <meeke...@gmail.com> wrote:

We've been over this many many times before as I'm sure you remember, and yet you STILL bring out that same silly argument even though you must know by now it's invalid.

Then explain it, instead of just asserting it.  Do you have a reference?

 

1) Many Worlds does NOT insist that the number of worlds is equal to the number of points in a line. 

2) Many Worlds does NOT insist that the number of worlds is equal to the number of countable integers.

3) Many Worlds does NOT insist that the number of worlds is an infinite number at all.

So you say.  But it is an implication of the fact that the probability of decay per unit time is constant. 

Brent

[John Clark]

On Sun, Aug 10, 2025 at 12:52 AM Brent Meeker <meeke...@gmail.com> wrote:

>> We've been over this many many times before as I'm sure you remember, and yet you STILL bring out that same silly argument even though you must know by now it's invalid.

> Then explain it, instead of just asserting it. 

As I've said, you've asked me to explain that many many times before, and I've explained it many many times before. If you think you've found a flaw in some part of my explanation then point it out and then I'll either explain it better, fix the flaw, or admit that you found a problem with Many Worlds that I hadn't considered before. But don't keep asking me the exact same question over and over again. 

> Do you have a reference?

Try "Something Deeply Hidden" by Sean Carroll, it's a wonderful book, one of the best I've ever read and I read a lot. You can get it on Amazon: 

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime  

>> 1) Many Worlds does NOT insist that the number of worlds is equal to the number of points in a line. 
2) Many Worlds does NOT insist that the number of worlds is equal to the number of countable integers.
3) Many Worlds does NOT insist that the number of worlds is an infinite number at all.

> So you say. 

So says every serious advocate of Many Worlds on the planet!  And the same thing could be said about every serious opponent of Many Worlds. And both previous statements remain true even if you include #4 which for some reason you left out:

"4)  Many Worlds does NOT say that probabilities can be obtained by counting branches."

Maybe Many Worlds will turn out to be wrong, but if it is it won't be because of one of those four things. 

 

>But it is an implication of the fact that the probability of decay per unit time is constant. 

Not if there are only a finite number of units of time, and nobody knows if time can be divided up into infinitely many different infinitely small pieces or not, Many Worlds is agnostic on the subject.  If it's found out that time can be divided up that way then that would be the first time that science, as opposed to pure mathematics, has found an infinite number of ANYTHING.

 John K Clark    See what's on my new list at  Extropolis

4f2 

[Brent Meeker]

On 8/10/2025 2:47 AM, John Clark wrote:

On Sun, Aug 10, 2025 at 12:52 AM Brent Meeker <meeke...@gmail.com> wrote:

> Do you have a reference?

Try "Something Deeply Hidden" by Sean Carroll, it's a wonderful book, one of the best I've ever read and I read a lot. You can get it on Amazon: 

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime  

I read it when it first came out.  He doesn't address the question of continuous time implying a continuum of worlds.  

Brent

[John Clark]

On Mon, Aug 11, 2025 at 12:54 AM Brent Meeker <meeke...@gmail.com> wrote:

> Do you have a reference?

Try "Something Deeply Hidden" by Sean Carroll, it's a wonderful book, one of the best I've ever read and I read a lot. You can get it on Amazon: 

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime  

> I read it when it first came out.  He doesn't address the question of continuous time implying a continuum of worlds.  

You must not read that book very closely because he clearly says  he doesn't know if there are any Infiniti of worlds or not and Many Worlds does not need an Infiniti of worlds. Here is a quotation from "Something Deeply Hidden":

“We don’t know how often branching happens, or even whether that’s a sensible question to ask. It depends on whether there are a finite or infinite number of degrees of freedom in the universe, which is currently an unanswered question in fundamental physics. But we do know that there’s a lot of branching going on…”

I think you need to read that book again! 

 John K Clark    See what's on my new list at  Extropolis

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